The Expansion of the Universe is Faster than Expected (2001.03624v1)

Abstract: The present rate of the expansion of our Universe, the Hubble constant, can be predicted from the cosmological model using measurements of the early Universe, or more directly measured from the late Universe. But as these measurements improved, a surprising disagreement between the two appeared. In 2019, a number of independent measurements of the late Universe using different methods and data provided consistent results making the discrepancy with the early Universe predictions increasingly hard to ignore. We review key advances realized by 2019: -- The local or late Universe measurement of the Hubble constant improved from 10% uncertainty twenty years ago to under 2% by the end of 2019. -- In 2019, multiple independent teams presented measurements with different methods and different calibrations to produce consistent results. -- These late Universe estimations disagree at 4$\sigma$ to 6$\sigma$ with predictions made from the Cosmic Microwave Background in conjunction with the standard cosmological model, a disagreement that is hard to explain or ignore.

• The paper reveals that late Universe Hubble constant measurements exceed CMB predictions by 4σ to 6σ.
• It employs multiple independent methodologies, including Cepheid variables, Type Ia supernovae, and strong gravitational lensing.
• The findings suggest potential new physics, such as Early Dark Energy, may explain the observed expansion discrepancies.

The Expansion of the Universe: Discrepancies in the Hubble Constant

The paper under review offers a detailed examination of the significant discrepancies observed in measurements of the Hubble constant ($H_0$), which quantifies the present rate of expansion of the Universe. Astrophysical observations have improved measurement precision significantly over the last few decades, reducing uncertainty from 10% twenty years ago to under 2% by 2019. This improvement has highlighted a measurable tension between the late Universe estimations of $H_0$ and those predicted by the Cosmic Microwave Background (CMB) in conjunction with the Lambda Cold Dark Matter (ΛCDM) cosmological model for the early Universe.

Key Observations and Results

1. Independent Measurements and Consistency: Multiple independent research teams have contributed to reducing the uncertainty in late Universe measurements using different methodologies and calibrations, achieving consistent results that exhibit a significant statistical discrepancy—ranging from 4σ to 6σ—from predictions derived from CMB observations.
2. Early vs. Late Universe Discrepancy: The common value for $H_0$ estimated from CMB measurements with ΛCDM is approximately 67.4 km/s/Mpc. However, late Universe measurements employing various methods—such as the SH0ES collaboration which uses Cepheid variables and Type Ia supernovae as cosmic distance ladders—indicate values in the low 70s km/s/Mpc range, highlighting a misalignment with the early Universe predictions.
3. Alternative Methodologies and Measurements: Apart from traditional Cepheid and supernova methods, other techniques such as strong gravitational lensing, the tip of the red giant branch (TRGB) method, and measurements using Mira variables and water masers have been utilized to ascertain $H_0$. Each approach consistently yields late Universe measurements that exceed early Universe predictions.

Implications and Theoretical Considerations

The persistent discrepancy in $H_0$ measurements presents a challenging inconsistency that cannot be easily attributed to a methodological error, calling into question our understanding of fundamental cosmological principles. One scenario that may explain this discrepancy is the presence of new physical phenomena not encapsulated by the ΛCDM model. For example, the introduction of an additional component such as Early Dark Energy (EDE) could reconcile some differences by altering the Universe's expansion dynamics shortly before the CMB emerged.

The presence of a uniformly distributed large-scale void or variations in dark energy dynamics has been suggested but lacks substantial empirical support. Such hypotheses indicate the potential for new insights into cosmological mechanics that extend beyond current models.

Future Directions

This ongoing tension prompts the need for more precise measurements and the development of additional methodologies to cross-verify existing results. Observations using gravitational lensing and CMB data from ground-based polarization cameras are anticipated to provide further constraints on $H_0$ and help clarify this cosmic conundrum. The resolution of this discrepancy may yield a deeper understanding of the Universe's composition, potentially offering insight into the nature of dark matter, dark energy, and the fundamental physical laws governing cosmological evolution.

In conclusion, while current standard cosmological models robustly explain many observed phenomena, the noted discrepancies in the Hubble constant reflect a significant area of inquiry within the field. Progress in this area may well refine or redefine our understanding of the Universe, serving as a catalyst for theoretical advancements in cosmology.